In the case of a static interferometer, in order to obtain an intra-pupillary variation in the optical path difference, an existing solution consists in using mirrors, termed “reflecting mirrors”, comprising stairs. The reflecting mirrors allow the reflection of the incident beams, previously divided by a splitter plate, so that they are recombined at the output of the interferometer. During recombination, the interference fringes which form exhibit greater or lesser contrast according to the performance of the interferometer and of the angular size of the analysed field.
FIG. 1 represents a case of an embodiment of a prior art staircase interferometer comprising a splitter plate 7 dividing the incident beam originating from a source S into a first beam 4 reflecting on a mirror M2 and into a second beam 3 reflecting on a staircase mirror M1. The two reflected beams 3′ and 4′ recombine after a second visit to the splitter plate 7. The path difference, denoted OPDj, equal to the optical path difference between the waves 1 and 2 at the output of the interferometer, depends on the distance Δj, separating the stair j from the reference plane 5 corresponding to the zero optical path difference.
This solution also makes it possible to avoid any displacement of the reflecting mirrors by an often imposing device.
For each stair, the optical path difference varies as a function of the field angle, inducing a drop in the contrast of the interference fringes in the case of an extended source. This is the phenomenon of self-apodization. This phenomenon limits the maximum field admissible by the interferometer and therefore the instrument's field of view.
The objective of the field compensation is to cancel or to reduce the dependence of the optical path difference, more generally denoted OPD, on the field angle.
Field compensation has been studied in the case of Michelson interferometers for which the variation in the optical path difference is obtained by scanning the position of a mirror of one of the arms of the interferometer along the optical axis.
The principle consists in inserting into one of the arms of the interferometer a plate of thickness, denoted e, and of index, denoted n.
FIG. 2 represents such a device when a mirror M1 is translated by a position Δ with respect to a reference plane.
The incident beams originating from the source S are split on a splitter plate 7 into two beams 3, 4 reflecting respectively on a first mirror M1 and on a second mirror M2.
The optical path difference, denoted δ12, between the waves which interfere 3′, 4′ may then be written as a function of the field angle θ:
            δ      12        ⁡          (              e        ,        Δ        ,        θ            )        ≈      2    ·          {                                    (                          n              -              1                        )                    ·          e                +        Δ        +                                            θ              2                        2                    ·                      (                                                                                n                    -                    1                                    n                                ·                e                            -              Δ                        )                              }      
For a position Δ1 of the mirror M1 with respect to the reference plane, there therefore exists a plate thickness e1 making it possible to cancel the dependence of the optical path difference δ12 on the field angle θ:e1=Δ1·n/(n−1)
The optical path difference δ12(e1, Δ1) may then be written:δ21≈2·(n+1)·Δ1 
This optical path difference is not zero and does not depend on the field angle. There is compensation of the field effect on the interferogram.
A drawback of such a solution is that the device operates on condition that the optical path difference is constant in the plane of the pupil. Furthermore, this device must be adjusted at each position Δ of the mirror M1.
This solution therefore does not work in the case of a staircase-type static interferometer such as represented in FIG. 1. Indeed, the optical path difference varies in the plane of the pupil, each stair being placed at a position Δj which varies from one stair to another.
An aim of the invention is to alleviate the aforementioned drawbacks.